Oil and gas wells are conventionally drilled with drill pipe to a certain depth, then casing is run and cemented in the well. The operator may then drill the well to a greater depth with drill pipe and cement another string of casing. In this type of system, each string of casing extends to the surface wellhead assembly.
In some well completions, an operator may install a liner rather than another string of casing. The liner is made up of joints of pipe in the same manner as casing. Also, the liner is normally cemented into the well. However, the liner does not extend back to the wellhead assembly at the surface. Instead, it is secured by a liner hanger to the last string of casing just above the lower end of the casing. The operator may later install a tieback string of casing that extends from the wellhead downward into engagement with the liner hanger assembly.
When installing a liner, in most cases, the operator drills the well to the desired depth, retrieves the drill string, then assembles and lowers the liner into the well. A liner top packer may also be incorporated with the liner hanger. A cement shoe with a check valve will normally be secured to the lower end of the liner as the liner is made up. When the desired length of liner is reached, the operator attaches a liner hanger to the upper end of the liner, and attaches a running tool to the liner hanger. The operator then runs the liner into the wellbore on a string of drill pipe attached to the running tool. The operator sets the liner hanger and pumps cement through the drill pipe, down the liner and back up an annulus surrounding the liner. The cement shoe prevents backflow of cement back into the liner. The running tool may dispense a wiper plug following the cement to wipe cement from the interior of the liner at the conclusion of the cement pumping. The operator then sets the liner top packer, if used, releases the running tool from the liner hanger, and retrieves the drill pipe.
A variety of designs exist for liner hangers. Some may be set in response to mechanical movement or manipulation of the drill pipe, including rotation. Others may be set by dropping a ball or dart into the drill string, then applying fluid pressure to the interior of the string after the ball or dart lands on a seat in the running tool. The running tool may be attached to the liner hanger or body of the running tool by threads, shear elements, or by a hydraulically actuated arrangement.
In another method of installing a liner, the operator runs the liner while simultaneously drilling the wellbore. A drill bit is located at the lower end of the liner. This method is similar to a related technology known as casing drilling. One option is to not retrieve the drill bit, rather cement it in place with the liner. If the well is to be drilled deeper, the drill bit would have to be a drillable type. This technique does not allow one to employ components that must be retrieved, which might include downhole steering tools, measuring while drilling instruments and retrievable drill bits. Retrievable bottom hole assemblies are known for casing drilling, but in casing drilling, the upper end of the casing is at the rig floor. In typical liner drilling, the upper end of the liner is deep within the well and the liner is suspended on a string of drill pipe. In casing drilling, the bottom hole assembly can be retrieved and rerun by wire line, drill pipe, or by pumping the bottom hole assembly down and back up. With liner drilling, the drill pipe that suspends the liner is much smaller in diameter than the liner and has no room for a bottom hole assembly to be retrieved through it. Being unable to retrieve the bit for replacement thus limits the length that can be drilled and thus the length of the liner. If unable to retrieve and rerun the bottom hole assembly, the operator would not be able to liner drill with expensive directional steering tools, logging instruments and the like, without planning for removing the entire liner string to retrieve the tools.
If the operator wishes to retrieve the bottom hole assembly before cementing the liner, there are no established methods and equipment for doing so. Also, if the operator wishes to rerun the bottom hole assembly and continue drilling with the liner, there are no established methods and equipment for doing so. Some liner drilling proposals involve connecting a bottom hole assembly to a string of drill pipe and running the drill pipe to the bottom of the liner. Retrieving the drill string at the conclusion of the drilling would retrieve the bottom hole assembly.
One difficulty to overcome in order to retrieve and rerun a bottom hole assembly during liner drilling concerns how to keep the liner from buckling if it is disconnected from the drill pipe and left in the well. If the liner is set on the bottom of the well, at least part of the drilling bottom hole assembly could be retrieved to replace a bit or directional tools. But, there is a risk that the liner might buckle due to inadequate strength to support its weight in compression.